Yo! So, I've finally decided to come to terms with this idea...using sound to define manual or automatic gearboxes.
The idea came to mind when I was listening to my Kuruma accelerate in GTA Online, no radio, hardly any traffic, and I noticed a tiny clunk sound between every single gear (before the engine became too loud to hear it). I think this would be great, because I've noticed, for example, the T20 doesn't have this, I can hear what sounds to be a dual clutch - flaps behind the steering wheel. Other cars just have CVT (Surge, Dilettante), whereas a lot have this manual gearbox sound applied.
Here's two cars that I recorded a while back:
Notice the clunk? I've noticed it a lot in switching from reverse to neutral/first (do cars even have neutral in gta? xD). But for it to be a proper manual, you've got to hear a sound from first to second, second to first, etc (otherwise the sound could just be the gearbox of an auto - P,R,N,D,2,L).
It's great. I'll be doing the tests in other games to, as I reckon I've heard this in GTA IV (not too sure about 3d universe, maybe even that?).